In a refrigeration air-conditioning system such as a refrigerator-freezer, an air conditioner, and a heat pump type water heater, a vapor compression type refrigeration cycle using a rotary compressor is used.
In light of preventing global warming and so on, energy-saving and efficiency-enhancing measures are needed for the vapor compression type refrigeration cycle. As a vapor compression type refrigeration cycle that aims to provide energy-saving and efficiency-enhancing measures, an injection cycle using a two-stage compressor may be pointed out. To encourage increased use of the injection cycle using the two-stage compressor, cost reduction and further enhancement of efficiency are needed.
Further, due to tightening of regulations for reducing the global warming potential (GWP) of refrigerants, consideration is being given to use of a natural refrigerant such as HC (isobutane, propane), a low-GWP refrigerant such as HFO1234fy, and so on.
However, these refrigerants operate at a lower density compared to a chlorofluorocarbon refrigerant conventionally used, so that large pressure losses occur in a compressor. Thus, there are problems when these refrigerants are used. The problems are that the efficiency of the compressor is reduced, and that the capacity of the compressor is increased.
In a prior art refrigerant compressor, when a discharge valve that controls opening/closing of a discharge port opens, a refrigerant compressed at a compression unit is discharged from a cylinder chamber of the compression unit through the discharge port into a discharge muffler space. In the discharge muffler space, pressure pulsations of the refrigerant discharged therein are reduced, and the refrigerant passes through a communication port and a communication flow path and flows into an internal space of a closed shell.
At this time, over-compression (overshoot) losses occur in the cylinder chamber due to pressure losses occurring from the time of discharge from the cylinder chamber until entry into the internal space of the closed shell, and due to pressure pulsations caused by a phase shift between change in cylinder chamber volume and opening/closing of the valve.
In a two-stage compressor, a refrigerant compressed at a low-stage compression unit is discharged into a low-stage discharge muffler space. In the low-stage discharge muffler space, pressure pulsations of the refrigerant discharged therein are reduced, and the refrigerant passes through an interconnecting flow path and flows into a high-stage compression unit. That is, the two-stage compressor is generally configured such that the low-stage compression unit and the high-stage compression unit are connected in series by an interconnecting portion such as the low-stage discharge muffler space and the interconnecting flow path.
At this time, in the prior art two-stage compressor, large intermediate pressure pulsation losses occur due to additional characteristic causes such as (1), (2) and (3) below. The intermediate pressure pulsation losses correspond to a sum of over-compression (overshoot) losses occurring in the cylinder chamber of the low-stage compression unit and under-expansion (undershoot) losses occurring at a cylinder suction portion of the high-stage compression unit.
(1) A difference in the timing of discharging the refrigerant by the low-stage compression unit and the timing of drawing in the refrigerant by the high-stage compression unit causes pressure pulsations at the interconnecting portion, thereby increasing losses due to pressure pulsations in the cylinder chamber.(2) A difference in the timing of discharging the refrigerant by the low-stage compression unit and the timing of drawing in the refrigerant by the high-stage compression unit causes disruption to a flow of the refrigerant from a discharge port for discharging the refrigerant from the low-stage compression unit into the low-stage muffler space toward a communication port for passing the refrigerant flowing into the interconnecting flow path leading to the high-stage compression unit, thereby increasing pressure losses.(3) Pressure losses are increased because the interconnecting flow path is narrow and long, or because a connecting port (inlet/outlet) between the interconnecting flow path and a large space causes the flow of the refrigerant to shrink or expand, or because a three-dimensional change occurs in the flow direction of the refrigerant passing through the interconnecting flow path.
Patent Document 1 discusses a two-stage compressor configured such that the volume of an interconnecting portion is greater than the excluded volume of a compression chamber of a high-stage compression unit. In this two-stage compressor, the large-volume interconnecting portion serves as a buffer, thereby reducing pressure pulsations.
Patent Document 2 discusses a two-stage compressor including an intermediate container in which an internal space is divided into two spaces by a partition member.
One of the two spaces is a main flow space which communicates from a refrigerant discharge port of a low-stage compression unit to a refrigerant suction port of a high-stage compression unit. The other space is a reverse main flow space which is not directly connected with the refrigerant discharge port of the low-stage compression unit and the refrigerant suction port of the high-stage compression unit. A refrigerant flow path is provided in the partition member dividing the main flow space and the reverse main flow space, so that the refrigerant passes between the main flow space and the reverse main flow space through the refrigerant flow path.
In this two-stage compressor, the reverse main flow space serves as a buffer container, thereby reducing pressure pulsations in the intermediate container.
Patent Document 3 discusses a two-stage compressor in which an interconnecting flow path is configured by a flow path that passes in an axial direction through a lower bearing portion, a cylinder constituting a low-stage compression unit, and an intermediate plate dividing the low-stage compression unit and a high-stage compression unit. In this two-stage compressor, the interconnecting flow path is positioned in a closed shell for downsizing.
Patent Document 4 discusses a twin rotary compressor in which two compression units connected in parallel are provided as upper and lower units. In this twin rotary compressor, a barrier portion is provided in a lower muffler space so as to form a stagnation space separated from other area by the barrier portion. In this twin rotary compressor, a refrigerant path is formed in the lower muffler space from near a discharge port toward a communication port serving as a refrigerant gas outlet to an upper side space in a closed container.
Non-Patent Document 1 discusses a bent guide flow path for reducing a fluid resistance in a bent pipeline or a bent duct, such as an elbow or a bend. In particular, it is stated at page 77 of Non-Patent Document 1 that for a bend having a rectangular cross-section, the greater the curvature of the bend, the smaller the pressure loss coefficient (pressure loss coefficient (CP)=total pressure loss (ΔP)÷dynamic pressure (ρu2/2)). It is also stated at page 80 of Non-Patent Document 1 that the pressure loss coefficient is reduced when a bent pipe is configured with consecutive elbows. At page 82 of Non-Patent Document 1, effects of a bend having a rectangular cross-section and including guide blades are stated. It is stated therein that an elbow bending at a right angle has a large pressure loss coefficient so that the pressure loss coefficient is reduced by providing guide blades in the bend as appropriate.
An object having a blunt side and a sharp side to a flow characteristically has greatly varying resistance coefficients depending on the orientation to the flow.
For example, Non-Patent Document 2 shows the following equation for a resistance coefficient (CD) of a three-dimensional object: Resistance coefficient (CD)=resistance (D)÷dynamic pressure (ρu2/2)÷projected area (S)
It is also stated in Non-Patent Document 2 that resistance coefficients vary for the same hemispherical shape. When a convex side of the hemispherical shape is directed upstream of the flow, the resistance coefficient is 0.42. On the other hand, when the convex side of the hemispherical shape is directed downstream of the flow, the resistance coefficient is 1.17, i.e., approximately tripled. When a convex side of a hemispherical shell is directed upstream of the flow, the resistance coefficient is 0.38. On the other hand, when the convex side of the hemispherical shell is directed downstream of the flow, the resistance coefficient is 1.42, i.e., approximately quadrupled. When a convex side of a two-dimensional half-cylindrical shell is directed upstream of the flow, the resistance coefficient is approximately 1.2. On the other hand, when the convex side of the two-dimensional half-cylindrical shell is directed downstream of the flow, the resistance coefficient is 2.3, i.e., approximately doubled.
Non-Patent Document 2 (p. 446) also discusses about the resistance coefficient of a two-dimensional square cylinder and how the resistance coefficient changes depending on an angle of attack (α) to the flow. The resistance coefficient is highest at CD=2.0 when the bluntest side is directed upstream of the flow (α=0°, S=S0). The resistance coefficient is CD=1.5 when the sharp convex side is directed upstream of the flow (α=45°, S=1.41S0). When the angle of attack is increased in a range of 0° to 45°, the CD coefficient decreases to a minimum value of 1.25 at a limit angle (α=13°, 1.2S0) where separation occurs from the lateral side of the square. Then, the CD coefficient increases up to CD=1.5. The projected area increases gradually in a range of S0 to 1.41S0, but the pressure resistance reaches the minimum at the limit angle (α=13°).
Thin plates, thin airfoils, and airfoils are objects in which the resistance coefficient varies the most depending on the angle of attack (α) to the flow.
For example, givenResistance coefficient(CD)=resistance(D)÷dynamic pressure(ρu2/2)÷airfoil surface area(S),an object of two-dimensional airfoil shape generally has the smallest resistance coefficient at near zero angle of attack (α). The resistance coefficient remains nearly constant in a range of −5°<α<+5°. When the angle of attack is increased further, separation occurs from the upper airfoil surface at approximately 10°, where the resistance coefficient increases sharply.
According to thin airfoil theory, such characteristics also apply to symmetric airfoils such as circular arcs or elliptical arcs.
When a resistance (D) is present in a flow path of a width y, the resistance (D) is obtained by a difference between the amounts of momentum integrated at an inlet (I) and an outlet (O) of a flow path inspection face as follows:Resistance(D)=∫(pI+ρIuI2)dy−∫(pO+ρOuO2)dy 
Assuming that density (ρ) and velocity (u) are constant at the inlet and outlet of the flow path inspection face, the resistance (D) can be expressed to be equal to an integral of a pressure loss (ΔP) occurring in the flow path on the flow path width y, as shown below.Resistance(D)=∫(pI−pO)dy=∫(ΔP)dy Conversely, the pressure loss (ΔP) occurring in the flow path can be considered to be approximately proportional to the resistance (D) of an object placed in the flow path.